Closed Loop Position Controller


Closed Loop Position Controller



In a closed loop position controller system, the positional information from an output potentiometer (Po) which is mechanically coupled to a motor is fed back to a control amplifier. Then, the reference position input from the input potentiometer (Pi) is combined with the feedback signal at the input of the amplifier which drives the motor in proportion to the difference between two signals. When the two positions are identical, the output of the amplifier becomes zero. A simplified system diagram of a closed loop position controller which will be used in this experiment is shown in Figure 2.

Figure 2: Close loop position controller

There are three amplifiers in Figure 2. The A1 is an error signal generator, A2 is an error signal amplifier and A3 is the driver for the motor M. As Pi is turned away from Po, the difference between two potentiometers voltages become an error signal which appears at the input of A1. The error signal is further amplified through A2 and A3, and drives the motor in the direction to reduce the error voltage between Pi and Po. Therefore, as Pi is turned clockwise, Po follows the same direction. This feedback action continues until the output of A1 is reduced to zero. At this point, the voltage measured at Pi and Po are same but in opposite polarity. For example, if Pi is at +3V, then Po is at -3V, making the sum of two zero.

The final relative position between Pi and Po depends upon the gain of the amplifiers. For a large gain, the position of Po can be almost equal to the position of Pi. But when the gain is not sufficient, there can be an offset in the relative position. This offset is the “deadband” for a position controller.

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